The present invention relates generally to a method for reducing the thermal stress in heat exchanger components and/or tube plates. More particularly the invention provides a method for increasing the thermal difference between the fluids in heat exchanger sections or compartments without increasing the thermal stresses in the heat exchanger metal components. Thermal stress causes premature or unplanned fatigue failure, the most common service failure in heat exchangers. Heat exchanger design for high temperatures has considered thermal stress for many years set out in our above referred to Provisional Application. Further the ASME Pressure Vessel Code requires consideration of temperature gradients during vessel design. Thermal stress is not the same as very high or very low temperature protective means, such as insulation shrouds or radiation shields. When fluids of different temperatures are separated by a metal component within the heat exchanger, a temperature gradient is established within or across the metal component. In the particular case of cryogenic fluid heat exchangers high thermal gradients regularly occur. Since metals generally expand or contract in a fixed proportion as its temperature is increased or decreased, the level of temperature difference between one side of the metal and the other established by the different fluid temperatures results in one side of the metal plate or sheet expanding or contracting an amount different from the other side which results in thermal stress in the metal component. Since most heat exchangers contain fluids under pressure, which result in mechanical stress within the metal plate, the thermal stress may add or subtract from the total stress within the metal plate during start-up or during operation. Stress in metal plates result in plate deformation and too high a stress may lead to rupture, creep or failure by cyclical fatigue. Reduced plate deformation in and of itself is also desirable (ref. U.S. Pat. No. 5,518,066 for example) and is a cause for detrimental leakage at flanged joints.
Many types of mechanical design techniques have been employed to reduce detrimental thermal stress in heat exchangers such as described in Reference [3]. In some instances, ceramic coatings have been applied directly to the metal in a thin layer for corrosion and total thermal protection. In other instances, intermediate fluids are used to reduce individual temperature differences. The recommended level of temperature difference across metal components is between 100° F. and 200° F. The design allowable difference being generally determined by consideration of the type metal, the type fluid, the fluid velocity and component part being considered. For example, austenitic stainless steel has thermal shock susceptibility 3 to 6 times higher than carbon steel, and therefore temperature gradients are an important consideration for evaluation of fatigue life of austenitic S.S.
The teachings of the instant invention are particularly applicable to the vaporization of cryogenic fluids at temperatures to below −300° F. using steam or water, which may be at +50° F. to 400° F. The total temperature difference between the fluids is 350 to 700° F., hence the temperature difference in a metal component separating the two fluids is 350 to 700° F. Since this is well above the recommended difference of 100 to 200° F. in the cited references, high thermal stress can be expected, especially in the tube plate or sheet. Additionally, since the cryogenic fluid enters the tubes of the tube plate at high velocity, high thermal stresses are set up within the metal ligaments between adjacent tube holes within the tube plate because the high velocity at these locations creates functional heat, which in turn reduces the normal temperature difference in the fluid boundary layer. Further, austenitic stainless steel is a preferred metal of construction for cryogenic heat exchangers. The higher thermal stress potential of austenitic stainless steel affects the higher thermal shock susceptibility of the austenitic stainless steel. It is understandable, therefore that failures in these cryogenic heat exchangers is common in areas of high temperature difference in combination with high mechanical stress, especially in tube sheets and at the intersections of components attached to the tube sheets.
In any location within the heat exchanger, it is desirable and many times essential to reduce the temperature differences across the metal component.